The present invention relates to an induction heating device for inductively heating an object to be heated which is formed of conductive material.
The invention also relates to an induction heating fixing device of induction heating type for fixing to a sheet a toner image formed on the sheet while conveying the sheet.
The invention also relates to an image forming apparatus having an image forming unit for forming a toner image on a sheet and an induction heating fixing device of induction heating type for fixing to the sheet the toner image formed on the sheet while conveying the sheet having the toner image formed thereon by the image forming unit. Among image forming apparatus of this type are copying machines, laser printers, facsimiles and the like, typically.
In a typical fixing device of induction heating type, region which is heated by exciting coils with respect to axial direction of heating roller (corresponding to width direction of sheet) (the region will be referred to as “first heating width”) is determined in accordance with the sheet having the largest width that is fed to the device. That is intended for achieving satisfactory fixing over the whole area of the sheet having the largest width. In an example of FIG. 15, the sheet having the largest width is a paper form of A3 size and the largest width is represented as A3 W. When a sheet (a paper form of B4 size of which width is represented as B4W, in the example of FIG. 15) having a width smaller than the sheet having the largest width is fed, there is produced a part L2 in the first heating width, which does not contribute to heating of the sheet. Then the temperature of the part L2 becomes higher than the temperature of the part L1 that contributes to heating of the sheet, and the temperature of the heating roller varies with respect to width direction of sheet.
As a countermeasure against such temperature increase at ends of the heating roller, there have been proposed heating rollers that contain magnetic cores extending in width direction of sheet and split into three sections and an exciting coil wound in layers around the magnetic cores along inside of the heating roller and that contain demagnetizing coils (canceling coils) wound around the magnetic cores at both ends and extending in direction perpendicular to the layer of the exciting coil, as disclosed in patent literatures (Japanese Patent Laid-Open Publication 2001-60490 and 2001-135470). When a sheet having the largest width is conveyed, the demagnetizing coils are opened by a switching circuit so as not to function. Then satisfactory fixing can be achieved over the whole area of the sheet having the largest width. When a sheet having a width smaller than the largest width is conveyed, the demagnetizing coils are closed by the switching circuit. Then at the ends of the heating roller with respect to the width direction of the sheet, a change of magnetic flux produced by the exciting coil causes not only an induced current (eddy current) in the heating roller but also back electromotive forces (and resultant currents) in the demagnetizing coils. Thus the temperature increase at the ends of the heating roller is prevented.
In such an arrangement in which the demagnetizing coils extend in the direction perpendicular to the layers of the exciting coil as in the patent literatures, however, the exciting coil and most of the demagnetizing coils (portions of the demagnetizing coils other than end portions on the side of the exiting coil) are so apart from each other that leakage flux (magnetic flux that is produced by the exciting coil and that does not contribute to the induced current in the heating roller) misses the demagnetizing coils, and effective function of the demagnetizing coils is thereby prohibited. In addition, there is a problem in that increase in vertical size of the magnetic cores results in enlargement of the device.